Susceptors are often used in microwave heating packages to enhance the browning and/or crisping of an adjacent food item. A susceptor is a thin layer of microwave energy interactive material (e.g., generally less than about 500 angstroms in thickness, for example, from about 60 to about 100 angstroms in thickness, and having an optical density of from about 0.15 to about 0.35, for example, about 0.17 to about 0.28), for example, aluminum, that, when exposed to microwave energy, tends to absorb at least a portion of the microwave energy and convert it to thermal energy (i.e., heat) through resistive losses in the layer of microwave energy interactive material. The remaining microwave energy is either reflected by or transmitted through the susceptor.
As shown schematically in FIG. 1, the layer of microwave energy interactive material (i.e., susceptor) 102 is typically supported on a polymer film 104 to define a susceptor film 106. In most conventional susceptor films, the polymer film comprises biaxially oriented, heat set polyethylene terephthalate, but other films may be suitable. The susceptor film is typically joined (e.g., laminated) to a support layer 108, for example, paper or paperboard, using an adhesive or otherwise, to impart dimensional stability to the susceptor film and to protect the layer of metal from being damaged. The resulting structure 110 may be referred to as a “susceptor structure”.
It is known that susceptor structures exhibit “self-limiting” behavior, that is, upon sufficient exposure to microwave energy, the susceptor film reaches a certain temperature and begins to form a crack or line of crazing. While not wishing to be bound by theory, it is believed that this crack or line of crazing propagates along a line of least electrical resistance through the conductive layer. As the crazing progresses and the cracks intersect one another, the network of intersecting lines subdivides the plane of the susceptor into progressively smaller conductive islands. As a result, the overall reflectance of the susceptor decreases, the overall transmission increases, and the amount of energy converted into sensible heat decreases.
This self-limiting behavior may be advantageous in particular heating applications where runaway heating of the susceptor would otherwise cause excessive charring or scorching of the food item and/or any supporting structures or substrates, for example, paper or paperboard. However, in other applications, it may desirable to limit or delay this behavior to ensure that the susceptor generates sufficient heat to be transferred to the adjacent food item to achieve the desired level of heating, browning, and/or crisping.
The present inventors postulated that since the layer of microwave energy interactive material is extremely thin, the performance of a susceptor may be highly sensitive to imperfections on the surface of the film, with a smoother polymer film surface providing greater heating longevity, and a rougher polymer film surface accelerating the self-limiting behavior of the susceptor structure. The present inventors further postulated that the topography of the polymer film could be tailored to control the rate and degree of crazing, and therefore, the self-limiting behavior, of a susceptor structure.
Standard biaxially oriented, heat set PET films typically used to form susceptor films have surface structures (e.g., strain-induced crystalline lamella and other surface features). Such structures generally cause the surface of the film to be rough and/or irregular. In some cases, the peak to trough surface roughness may be from about 40 to about 100 nanometers or greater. Therefore, when microwave energy interactive material is deposited using vacuum vapor deposition onto the surface of the polymer film by line of sight travel from the metal source, it typically does not form a uniform layer. Instead, the microwave energy interactive material is non-uniformly deposited on the surface with some areas having more and some areas less or even no deposition of microwave energy interactive material. As a result, the conversion of microwave energy into sensible heat is likewise non-uniform. While not wishing to be bound by theory, it is believed that complex resistive-capacitive circuits are formed in the conductive layer, with the areas completely or nearly void of conductive aluminum acting as capacitors. The routing of electrical current throughout the polymer film may be preferentially channeled to the paths (or circuits) of lowest resistance. The I2R power loss in low resistance circuits exceeds the power loss in immediately adjacent areas of higher resistance. As a result, low resistance circuits heat the biaxially oriented, heat set PET film above its heat set temperature, and the resulting orientation stress relief causes a crack to form in the film.
Plasma treatment has been widely used in a variety of applications for altering the surface of polymer films. While there are many forms and uses for subjecting materials to plasmas, plasma treatment generally consists of exposing the surface of a film to a glow discharge. The resulting plasma is a partially ionized gas consisting of large concentrations of excited atomic, molecular, ionic, and free-radical species. Excitation of the gas molecules is accomplished by subjecting the gas, which in the present invention is enclosed in a vacuum chamber, to an electric field, typically generated by the application of radio frequency (RF) energy. Free electrons gain energy from the imposed RF electric field, colliding with neutral gas molecules and transferring energy, dissociating the molecules to form numerous reactive species. It is the interaction of these excited species with films placed in the plasma that results in the chemical and physical modification of the film surface.
In many instances, the plasma treatment conditions are selected for the polymer film to provide a roughening of the surface that allows the film to receive other materials. For example, Ionita et al. (Ionita, R, M. D., Stancu, E. C., Teodorescu, M., Dinescu, G., “Small size plasma tools for material processing at atmospheric pressure”, Applied Surface Science 255 (2009) 5448-5482) exposes films to an argon plasma of 14 W power delivered by an 8 mm diameter probe traversing the film sample at 5 mm/s in ambient atmosphere (14 W, 0.2 s exposure/mm2, yielding 2.8 J/mm2 per pass or 14 J/mm2 or 1400 J/cm2 per 5 passes) (p. 5449). As another example, U.S. Pat. No. 7,579,179 to Bryhan et al. describes a plasma treatment up to 800 J/cm2 intended to significantly roughen surfaces to enhance biological cell growth and cell attachment. A large list of gases is described, some of which were applied at extremely high applied power to create significant roughness.
Plasma treatment has also been done under conditions in which little or no surface roughening occurred. For example, Beake et al. (Beake, B. D., Ling, J. S. G., Leggett, G. J., “Scanning force microscopy investigation of poly(ethylene terephthalate) modified by argon plasma treatment”, Journal of Materials Chemistry, 8(8) (1998) 1735-1742), biaxially oriented PET film was exposed to argon plasma at 0.1 mbar, 10 W power for 1, 10, 20, 60 and 90 minutes. Despite the clear differences in type of topography seen in FIGS. 2 and 3 of the article, the authors state “The topographical changes resulting from plasma treatment were not accompanied by a change in surface roughness, as measured by the variance of the RMS height of the surface features, which remained constant . . . very close to the value determined for the untreated Melinex ‘O’.” Beake et al. also report that in addition to their own experiments, Fischer et al. “have reported scanning electron microscopy (SEM) data showing that whilst oxygen plasma roughens the PET surface, argon plasma does not” (Fischer, G., Haeneyer, A., Dembowski, J., Hibst, H., “Improvement of adhesion of Co—Cr layers by plasma surface modifications of the PET substrate”, J. Adhes. Sci. Technol., 8 (1994) 151, see FIG. 2 showing that after 10 min etching time arithmetic mean roughness remained essentially the same as that of the untreated film).
Amanatides et al. (Amanatides, E., Mataras, D., Katsikogianni, M., Missirlis, Y. Y., “Plasma surface treatment of polyethylene terephthalate films for bacterial repellence”, Surface & Coatings Technology, 100 (2006) 6331-6335) report on average surface roughness changes after 15 minutes etching time using 80% He/20% O2 gas at 45.7 J/cm2 that “the PET films treated under negative bias have lower surface roughness compared to the ones treated with no bias” (see p. 6334).
Ardelean et al. (Ardelean, H., Petit, S., Laurens, P., Marcus, P., Arefi-Khonsari, F., “Effects of different laser and plasma treatments on the interface and adherence between evaporated aluminum and polyethylene terephthalate films: X-ray photoemission, and adhesion studies”, Applied Surface Science 243 (2005) 304-318) exposed PET films to 95% He/5% O2 plasma at a plasma treatment energy of 0.2 J/cm2 and report that at those conditions “the surface topography of the plasma treated surface showed no difference with the non-treated polymer” (p. 311).
Liston et al. (Liston, E. M., Martinu, L., Wertheimer, M. R., “Plasma surface modification for improved adhesion: a critical review”, J. Adhesion Sci. Technol. 7 (10) (1993) 1091-1127) state on p. 1097, “For example, plasma surface treatment of fluoropolymers for short times improves their wettability without modifying their surface texture, but overtreatment gives a very porous surface [27, 28]. The same is true for polyethylene terephthalate (PET)[29].” (where Reference 29 is Y.-L. Hsieh, D. A. Timm and M. Wu, J. Appl. Polym. Sci. 38, 1719-1737 (1989)).
It has also been recognized that plasma treatment may result in non-uniform ablation of topographical surface features, depending on the specific surface features and geometry of the film being treated. This phenomenon has been studied particularly in the area of MEMS (microelectromechanical systems). See, e.g., Volland, B. E., Heerlein, H., Kostic, I. and Rangelow, I. W., “The application of secondary effects in high aspect ratio dry etching for the fabrication of MEMS”, Microelectronic Engineering, 57-58 (2001) 641-650, and Kiihamaki, J., Kattelus, H., Karttunen, J., Franssila, S., “Depth and profile control in plasma etched MEMS structures”, Sensors and Actuators, 82 (2000) 234-238. As the authors indicate, several secondary effects are well known in plasma etching for MEMS fabrication—reactive ion etch lag (RIE-lag, small features etch slower than large features) and aspect ratio dependent etching (ARDE, greater aspect ratios of features create increasing shadowing effects, reducing etching rates in areas bounded by the features). Both impact uniformity of etch rates and hence material removal and thus impact the results of etching processes.
There is a continuing need for susceptor films that exhibit the desired level of crazing, and therefore, desired level of heating for a particular application. Although some attempts to understand the self-limiting behavior of susceptors have been made, the relationship between the surface characteristics of oriented films used for microwave susceptor films and the resulting susceptor performance has generally not been explored or understood. The present inventors have discovered that plasma treatment of films may be used to modify the behavior of susceptors to attain these desired properties. Various aspects, features, and embodiments will be apparent from the following description and accompanying figures.